Polytetrafluoroethylene aqueous paste and process for shaping same



POLYTETRAFLUOROETHYLVENE AQUEOUS PASTE AND PROCESS FOR SHAPING SAME du Pont de Nemours and Company, Wilmington, Del.,. a corporation of Delaware Application September 8,1954 Serial No. 454,845

3 Claims. 01. 260-17) No Drawing.

- This invention relates to thickened aqueous pastes of polytetrafiuoroethylene and to the manufacture of arti- C loidal dispersions of polytetrafiuoroethylene can be extruded into a coagulating medium to produce filaments,

tapes, films, etc. which on subsequent sintering become quite strong (Barry U.S. 2,559,750, issued July 10,1951). It has also been disclosed heretofore that polytetrafiuoroethylene can be molded by cold-pressing finely divided polymer to the desired shape and thereafter sintering the shaped article (Brubaker et al. U.S. 2,400,099, issued May 14, 1946, and Benning et a1. U.S. 2,400,094, issued May 14, 1946; cf. also the Fields Patent U.S. 2,456,262, issued December 14, 1948). More recently, it has been found that colloidal sized particles of polytetrafiuoroethylene, coated with a hydrocarbon lubricant or other organic lubricant which is capable of being removed by the action of heat, can be extruded and the extrudate,

2,881,142 Patented .Apr. .1959

hydrocarbon lubricants. The hydrocarbon-lubricated compositions were, however, inflammable, and the volatile components which were released during the processing of the hydrocarbon-lubricated compositions were a source of safety hazard unless appropriate precautions John Emerson Eldridge, Newark, NJ., assignor to E. I. wgre taken An object of this invention is to provide a process for preparing shaped articles from tetrafluoroethylene polymer pastes in which the lubricant is composed of water and added ingredients which permit the use of water in place of the previously used inflammable lubricants. Another object is to provide compositions suitable for use in making such shaped articles. A further object is to shape such compositions whereby shaped articles are formed without any hydrocarbon lubricant being required. A still further object is to extrude such compositions into filaments, tubes, coatings on wires, film and the like. Other objects of the invention will appear hereinafter.

The present invention is based in part upon the discovery that in the use of aqueous lubricants in colloidal polytetrafiuoroethylene pastes, the wettability of the unsheared particles by the aqueous lubricant, and the vis cosity of the lubricant, are dominant factors in obtaining good results. In contrast with hydrocarbons, water does not have the inherent property of wetting colloidal polytetrafiuoroethylene particles when such particles have after being freed of lubricant, can be sintered to form v, a tough product (Llewellyn and Lontz U. S. patent application S.N. 171,534, filed June 30, 1950, now U.S. Patent 2,685,707; Ind. Eng. Chem. 44, 1805-1810 (1952);. 45, l123-1127 (1953)). Quite generally, thickeners have been employed in hydrocarbon media in the be removed readily by the action of heat prior to or tent, this change in the character of the particles takes prevent melting of the polymer has been disclosed in 1 been subjected to shearing, as explained hereinafter. One

of' the significant features of the present invention, in

one of its particular embodiments, is the use ofadditives which not only affect the viscosity of the aqueous lubri cant but which also impart ability to wet the colloidal particles of polytetrafiuoroethylene. The viscosity of the lubricant is an important factor, of course, and should be controlled by the methods disclosed herein.

The wetting of the surfaces of polytetrafiuoroethylene particles depends not only on the nature of the liquid with which the surfaces are to be wet but also uponthe character of the polytetrafiuoroethylene particle itself. One of the characteristics of colloidal polytetrafiuoroethylene particles is their tendency to be converted to a fibrous form when rubbed together. This change in the nature of the particles greatly affects extrudability as the Hahn and Mallouk U.S. patent application S.N.

405,654, filed January 22, 1954, now U.S. Patent 2,770,- 842, theet'r'ect of pressure in restraining the melting of polytetrafiuoroethylene being disclosed in the Tordella U.S. patent application S.N. 375,822, which was filed on August 21, 1953, now U.S. Patent 2,791,806.

Attempts have been made to employ aqueous lubricants in the injection molding of polytetrafiuoroethylene in place of the hydrocarbon lubricants disclosed in the aforementioned. U.S. patent applications S.N. -'171,533

and SN. 171,534, but it hasbeen'found that-water as alubricant is not as effectiveas the hydrocarbon lubricants (including thickened hydrocarbon lubricants) which had given satisfactory results. The products obtained here tofore' through the use of water as a lubricant were not as strong as those which were obtained with the preferred well as the wettability of the particles by aqueous lubricants. Particles which have undergone this change can still be wet by hydrocarbon lubricants, and can be shaped in the presence of the latterlubricants. To a slight explace, in certain instances, upon merely subjecting the aqueous dispersion to vigorous agitation to produce a coagulation thereof (as disclosed in the Lontz Patent U.S. 2,593,583). For this reason, the removal of the particles from the polymerization mixture (produced by dispersion polymerization of tetrafiuoroethylene), when these particles are to be used in the process of this in vention, is preferably achieved without vigorous mechanical working. The shaping of articles from the waterwettable particles (which have not been mechanically worked) should be done under conditions which do not involve conversion of the particles to fibrous form in advance of actual shaping of the article. This does not preclude extruding the aqueous pastes herein disclosed, but it does seriously interfere with 'mo'lding articles'of extruded or otherwise worked polymer. Shaping metli-" ods which do not involve preliminarily converting poly tetrafiuoroethylene particles to a non-wettable form are, however, entirely suitable, using the aqueous thickened lubricants disclosedherein. a

were

Aftenthe polymer has been shaped, in the manner just 7 described,"it is freed"of.- water by drying at sufficie'ntly high temperature to permit rapid removal of the water. It is thereafter heated to sintering temperature to pro duce a shaped article of excellent strength, while at the same time decomposing the thickener component of the lubricant.

The polymers employed in the practice of this invention i nclude, polytetrafiuoroethylene and tetrafluoroethyleni'polymr. with end" groups. supplied by chain trailSfcf agentsfsuch as allianol's.

' The, additives'fw'hich are present in the. aqueous lubricantsi'ern'ployed in" the. 'pfactic'efbf this. invention may incliidematerialswhi'ch have'a thickening and wetting action. when. present in very small concentration. Water thickeners, which at small concentrations. are relatively ineffective forfproducing'the desired v'iscosityfand the required wetting effect, may be employed in high con: centration, but it is'disadvantageou's' to employ such niaterials'frequiring high' concentration, because they are rath r diflic'ult to remove from the final shaped article and'ma'y. decrease, the rate" at which 'it dries. In this cennection, it. is to be noted that 'an excessivelyfrapid rate of drying, i.e. an unduly high drying temperature, must be avoided, because of the tendency for such articles to form blisters when the rate of. drying is excessive; It is preferred 'to. employ as thickeners materials, such methyl cellulose ether, which are highly effective at minimum concentrations, i.e. which have the'desired effect upon wetting power and viscosity even when present to the extent of as little as 0.1% based on the weight of. polytetrafiuoroethylene. Such'materials are relatively easily removed from the final sintered article by burning ofi. (i.e. oxidizing with elemental oxygen) without leaving anyappreciable quantity. of char. In contrast with this, additives which must be present in relatively. high conf cenltjr ations (elg. 10% of. the quantity of polytetrafluoroethylene) to produce the desired etfect on wetting power and viscosity are rather. difficult to remove during sinter ingot in a subsequent heating step and produce articles which are black and contain relatively large amounts, of char. In some instances, the desired wetting power and the desired viscosity can be achieved by combining an agent which has the required viscosity-imparting property with a material which hasthe desired wetting agent.

Suitable examples of additives which are effective include methyl cellulose ether, polyvinyl alcohol, sodium carhoxy. methyl cellulose, polyethylene oxide, polyvinyl pyrrolidone, styrene/maleic anhydride copolymer, vinyl acetate/maleic anhydride copolymer half calcium salt. In the case of methyl cellulose ether, the quantity of thickener which gave excellent results could be as little as 1% of the quantity of polytetrafluoroethylene; in fact, as little as 0.1% was found to give good products. A preferied range is about 0.1 to 2.0%. With methyl eelhilcmse ether or polyvinyl alcohol, as well as with un merous other thickeners, it was not essential to employ a wetting agent. However, Triton" X-100 (which is an alkyl aryl polyether. alcohol non-ionic surface-active agent, more specifically a polyethylene glycol-p-octyh phenol ether made by Rohm and Haas Company) could be used to improve wettability with either of these com pounds or. with other thickeners.

The paste which is employed in the practice of this invention can be prepared by concentrating an aqueous dispersion of colloidal polytetrafiuoroethylene by any. of various means which are well known in the art (cf. also Marks and Whipple patent application SN. 348,116, filed April 10, 1953, now abandoned).

In one ofthe methods for practicing the invention, the 'thicken e'r is added to the concentrated dispersion (together with the wetting agent if desired), and the. resulting mixture is further concentrated by evaporation, or by coagulation followed by. decantation of water. Evaporation is the preferred method for. concentration at this stage because this minimizes any loss of agents whichenhance the viscosity and wetting power snare liquid phase. The evaporation can be carried out simply by maintaining the thickened dispersions on trays at temperatures of from room temperature to 120 C. In one method of preparing the paste, the evaporation is continued until the resulting residue can be lifted from the tray by means of a spatula and broken into small chips. These chips thus produced are free flowing and show no tendency to compact into larger lumps.

Another method of drying found to be feasible is to feed the. thickened dispersion into the nip of. two steam heated "rollsf'enc'losed by, an evacuated box. Doctor bladesar'e iised to scrape the dried polymer' front" the rolls. Where thefpr'oduct obtained is. too sticky for convenient handling, ammonium nitrate or carbonate can be added to reduce the sticking.

Another method of. drying Which is suitable for larger scale operation" is the of a horizontal endless belt upon which a layer of dry residue can be formed, the

residue being capable of cracking'oif-the belt as the belt passes over the pulley. The pieces of residue thus formed are readily broken into free flowing chips.

Tlidry', powder obtained" as above described is mixed with water to' produce an extrndable paste. If desired, the water, can bea'dded asa spray into the mixture which can simultaneously be tumbled in order to distribute the water, as uniformly as possible. Alternatively, the water can be'added in bulk and distributed throughout. the mixture by turnblin'gI It, was found to be helpful. to. allow -the mixture to stand in a closed containefioverlf ni'g'ht, beforeextrusion; for in' thisway the water, can penetrate; mere" Completely by. diffusion and capillary action into. the inte'ridrhfthe, larger chips; The quantity ciffiivdterf whichfisprsent in the pasteis prefrablyftlie; miniiriurh which is necessary to" produce suificientlubr'i' cation. Thisquantity of water can be as little as 5% and generally is not in excess of 40% based on the. weight of. po ymer. The. quantity of water used is determined: to so eextentfby the complexity of the desired shaped article. When the quantity of. aqueouslubric'ant is small, the driving force required to extrude the compo sition' i's, unduly, gieat. Good all-around performance. is. obtained from compositions containing about 18% water based on the total weight of polymer and other. ingredients 'of the'paste. Example IJ -Tc; an aqueous colloidal dispersion obtained by polymerizing tetrafluoroethylene is addedwith thorough mixing 1% by weight of methyl cellulose ether; based on the weight of polytetrafluoroethylene polymer. The, water 'contentof the resulting composition is re: ducedby'evaporation, using a shallow tray in an oven set at C., the depth of the mixture in the tray be i'ng initially inch. When a solid residue which can be lifted from the tray is obtained, this dry. agglomerated. colloidal residue is. removed and placed in a's'uita'ble container wherein it is thoroughly admixed with 18% of water. based on the weight of the polymer. The re; sulting paste was extruded readily through a die and. the extrudate, which was in the form of a pipe liner, was sintered'at 327v C. to give an outstandingly strong, although black, pipe liner suitable for use for its intended purpose.

In the process illustrated above, fillers, such as. finely. divided carbon, etc., may be present in the aqueous paste. prior to the shaping thereof.

The method illustrated in the example was appraised, with results, which were satisfactory from the standpoint of the formation of strong articles, using the thickeners, listed in the following table, The table also records the, behavior of the respective products. on heatingto above, the, sintering. temperature. The water-soluble cellulose, ether-s were, generally effective and constitute a preferred class. of thickeners. for usein the practice of the invention.

The thickened pastes hereinabove described are composed of a coagulum of colloidal particles, constituting a phase made up of contiguous particles, the lubricant being interspersed within said coagulum, i.e. adjoining the surfaces of the coagulum without preventing contact between particles. Water insoluble materials are not included in the term thickener as used herein, and, of course, coating compositions which do not contain water soluble thickeners, even though other resinous components are present (as in US. 2,668,157) are not within the scope of this invention.

The compositions described in the foregoing illustrations can be shaped by simple compression, upon application of pressures of 1000 to 2000 p.s.i., which on subsequent drying can be sintered to tough sheet stocks suitable for use in gaskets or electrical insulation such as slot liners. In addition, complex shapes such as flanged diaphragms can be compression-formed in molds having the desired contours by charging the lubricated powder or chips and applying pressures up to 2000 p.s.i., followed by drying and sintering. The pastes can be shaped in numerous other ways also, provided, as explained above, the formation of the fibrous form prior to shaping is avoided.

Pastes obtained as hereinabove described can be extruded satisfactorily to produce smooth, flexible extrudates. In general, the unsintered extrudates obtained from these pastes have somewhat less elongation but slightly higher tensile strengths than similar unsintered extrudates made from oil-lubricated pastes. There is very little difference in toughness between the two types of extrudates. At a water content of about 18%, the driving force necessary for extrusion is about the same as for the best oil-lubricated pastes; at water contents below 18%, the aqueous paste requires more driving force to extrude.

The extrudate must be dried carefully before sintering in order to avoid blistering.

After drying, the shaped articles are subjected to a sintering action at temperatures of at least 327 C. This results in the charring of the thickener since the thickener burns off during the sintering step. The black color produced by burning oil is not detrimental in certain applications, such as in pipe liners, but it may be objectionable in certain other applications. To eliminate chairing, the sintering can be conducted in a ventilated oven whereby the shaped articles lose most of their color when heated at sintering temperatures for prolonged periods of time, namely about 45 minutes to three hours (for approximately 0.060 inch thicknesses). This method is quite effective for decolorizing thin articles but it is generally not employed for decolorization of relatively thick walled articles.

Still another method for avoiding the formation of dark-colored products is to include a small quantity of ammonium nitrate in the paste prior to extrusion. When this is done, the sintering of the extrudate causes the salt to decompose and liberate oxygen which burns out the carbon residues quickly. A disadvantage of this method is that it results in the formation of products which are not as tough as those which are formed in the absence of soluble salts. Thus the method chosen for avoiding the discoloring produced by charting is determined by the nature of the product which is desired or the use to which it is to be put, and in fact, the need for avoiding formation of charred products also depends on the properties which are desired in the final product.

The aqueous pastes which are employed in the practice of this invention are composed of tetrafluoroethylene polymer particles of colloidal size admixed with the hereinabove described lubricant mixture having a viscosity of 10 to 10 poises, or more. A characteristic property of the lubricant mixture is its capacity to wet polytetrafiuoroethylene. The lubricant mixture is composed of water, and at least one organic additive including a thickener. The wetting power of the lubricant is enhanced by at least one of the said additives. One of the significant properties of the composition just described is the extrudability thereof. These compositions can be shaped as hereinabove described or extruded in other forms such as in the form of filaments or heading.

The shaped articles obtained in the practice of this invention can be used for a variety of applications including many of those for which shaped polytetrafluorw ethylene articles, made from oil-lubricated polytetrafluoroethylene compositions, can be used. Particular examples include liners for pipes or chemical equipment, tubular products, etc.

I claim:

1. A composition comprising a coagulum of tetrafluoroethylene polymer particles of colloidal size and from 5 .to 40% by weight of said tetrafluoroethylene polymer of a lubricant having a capacity to wet polytetrafluoroethylene, said lubricant being composed of water and at least one organic additive including from 0.1 to 10% by weight of said tetrafiuoroethylene polymer of a polymeric water-soluble organic thickener therefor, said lubricant being interspersed in said coagulum, said polymeric thickener being a member of the class consisting of methyl cellulose ether, polyvinyl alcohol, and sodium carboxymethyl cellulose.

2. A process for preparing shaped articles which comprises shaping at a temperature below the boiling point of water a paste having the composition set forth in claim 1, said paste being further characterized in that it is composed of a coagulum of unsheared colloidal particles of tetrafluoroethylene polymer and a lubricant having a viscosity of 10 to 10 poises, said lubricant having the capacity of wetting said tetrafluoroethylene polymer, said lubricant being composed of water and at least one organic additive including a dissolved polymeric water-soluble organic thickener therefor, wetting power of the said lubricant being enhanced by at least one of the said additives, the quantity of said lubricant being such that the lubricated mixture contains from 5 to 40 parts by weight of water per parts of said tetrafluoroethylene polymer, said lubricant being interspersed in said coagulum, said polymeric thickener being a member of the class consisting of methyl cellulose ether, polyvinyl alcohol, and sodium carboxymethyl cellulose.

3. Process of claim 2 wherein the said thickener is methyl cellulose ether and the quantity thereof is from 0.1 to 2.0% of the weight of polytetrafluoroethylene.

References Cited in the file of this patent UNITED STATES PATENTS Berry Aug. 9, 1949 

1. A COMPOSITION COMPRISING A COAGULUM OF TETRAFLUOROETHYLENE POLYMER PARTICLES OF COLLOIDAL SIZE AND FROM 5 TO 40% BY WEIGHT OF SAID TETRAFLUOROETHYLENE POLYMER OF A LUBRICANT HAVING A CAPACITY TO WET POLYTETRAFLUOROETHYLENE, SAID LUBRICANT BEING COMPOSED OF WATER AND AT LEAST ONE ORGANIC ADDITIVE INCLUDING FROM 0.1 TO 10% BY WEIGHT OF SAID TETRAFLUOROETHYLENE POLYMER OF A POLYMERIC WATER-SOLUBLE ORGANIC THICKENER THEREFOR, SAID LUBRICANT BEING INTERSPERSED IN SAID COAGULUM, SAID POLYMERIC THICKENER BEING A MEMBER OF THE CLASS CONSISTING OF METHYL CELLULOSE ETHER, POLYVINYL ALCOHOL, AND SODIUM CARBOXYMETHYL CELLULOSE. 